The present invention relates to a method and/or architecture for calibrating phase-shift networks generally and, more particularly, to high frequency or narrowband signal processing.
Referring to FIGS. 1a and 1b, a pair of high-frequency (or narrowband) signal processing applications 10 utilizing phase-shift networks are shown. The signal processing application 10 of FIG. 1a synthesizes (or generates) two signals with a specific, 90 degree (i.e., quadrature) phase relationship to each other. A narrowband input RF (i.e., a single sinusoid) is fed into the phase shift network 10 of FIG. 1a, which then outputs two desired quadrature signals of equal amplitude and 90 degree phase relationship. A second input to the phase-shift network 10 of FIG. 1a is coupled to ground.
The signal processing application 10 of FIG. 1b is an image-rejection mixer (found in cellular phones, wireless local-area networks, television tuners, etc.). An incoming radio-frequency (RF) signal is fed into two separate mixers, one driven by a sine wave, and the other driven by a cosine wave. The filter is typically implemented as a low pass filter (or as no filter at all). The output of each mixer has the classic problem of undesired image-band conversion, where not only the desired signal (i.e., 1 and 4) is converted into the mixer output, but also any signal at the associated image frequency (i.e., 2 and 3). The phase-shift network 10 of FIG. 1b combines the outputs of both mixers, due to the 90 degree phase relationship of the undesired image and the desired signal, the phase-shift network 10 of FIG. 1b can then separate the two signals, leaving only the desired signal.
In the phase-shift networks 10, the critical performance metric of the phase-shift network is in gain/phase matching. How well balanced the outputs of FIG. 1a, or how well separated the undesired image of FIG. 1b, is determined by how accurately the phase-shift network 10 can achieve the desired 90 degree phase relationship.
Referring to FIG. 2a, a conventional resistor-capacitor (RC) ladder network method for implementing a phase-shift network 20 is shown. Since the PSN 20 is implemented using only resistors and capacitors, integration in standard silicon is amenable. However, such PSN architectures have the drawback of only achieving precision 90 degree phase shifts at one specific frequency (i.e., xcfx89=1/(R*C)). Furthermore, typical applications only allow for a phase shift error of approximately 1 degree (i.e., the PSN must shift somewhere between 89 and 91 degrees). Thus, any on-chip variability will substantially limit the effectiveness of such a PSN. For example, a 500 MHz PSN meeting a 1 degree phase error specification would require on-chip resistors and capacitors accurate to better than 0.5%. Laser trimming can achieve the exceptionally high level of accuracy. However, laser trimming is expensive and difficult to implement (i.e.; inappropriate for high-volume consumer applications such as cellular phones).
Referring to FIG. 2b, a conventional cascaded multiple phase-shift network circuit 30 is shown. The cascaded PSN circuit 30 provides a range of frequencies over which accurate phase shift occurs. The circuit 30 allows for on-chip resistor and capacitor tolerances. Therefore, the circuit 30 is insensitive to on-chip variability. However, every additional stage of phase shift results in signal attenuation (since the circuit 30 is passive), as well as additional noise from the resistors. Thus, a designer is strongly motivated to utilize as few stages of phase-shift as possible, while meeting the performance requirements.
The present invention concerns an apparatus comprising a first calibration circuit and a phase shift network stage. The first calibration circuit may be configured to generate a control signal. The phase shift network stage may comprise one or more tunable phase shift elements and be configured to provide a tunable impedance. The phase shift network stage may be tuned in response to the control signal and a conductance of the tunable phase shift elements.
The objects, features and advantages of the present invention include providing a method and/or architecture for calibrating phase-shift networks that may (i) be automatically calibrated, (ii) be implemented without requiring laser trim, (iii) implement fewer stages and/or (iv) be implemented without other costly per-part manufacturing adjustments.